Which game is more important, Smash Bros or Mario Kart

The Wii U has fallen behind its competitors fairly quickly in terms of sales. Many people like to say it's because certain games aren't out yet. Two of which were Mario Kart and Smash Bros. Now, Mario Kart is already out. Smash Bros will be out in two weeks. But which game will be more of a boost for the system when all is said and done?